Objective Reviews & Commentary - An Engineer's Perspective

July 6, 2011

Cmoy With Gain

cmoy repairBACKGROUND: I recently reviewed a $39 Cmoy purchased direct from China on eBay and was somewhat impressed except for one obvious problem: It had a gain of “1X” or 0 dB which rendered it useless for most applications.

NOW WITH GAIN! Fixing the gain problem required replacing 4 resistors. The photo shows the original resistors removed and the 4 new ones laying above the Cmoy’s circuit board (click for larger version). The Tech Section goes into more detail on the resistor values, etc.

HISS & NOISE: Even using my most sensitive headphones, Ultimate Ears SuperFi Pro 5s, and even with 13 dB of gain, the Cmoy is still very quiet when driven from a low impedance source. Subjectively it’s even quieter than the Mini3 at some volume settings. There’s a “pop” when you first turn it on that’s not that bad—especially with more typical headphones. There’s a softer click when you turn it off.

SUBJECTIVE SOUND QUALITY: With a proper amount of gain the 4556 Cmoy sounds at least decent even with my Sennheiser HD650s. It has plenty of power for the 650s and also my 80 ohm Beyer DT770s using my iPod Touch or Sansa Clip+ as a source. Without the Cmoy, and the gain modification, neither source works well for those headphones. So the modified Cmoy is a big improvement. I might conduct some blind listening tests in the future to find out just how good a $39 amp can be.

MEASUREMENT SUMMARY: As expected from using less feedback there’s more distortion but the measurements are still respectable and better than the Mini3 in several areas. In particular, the Cmoy has a much higher output voltage than the Mini3 with roughly five times more power into high impedance loads. That could make a big difference with some power hungry high impedance cans. And the Cmoy’s real ground design is free of the serious interchannel distortion problems that plague the Mini3. It also has less distortion at high frequencies, a lower output impedance and much better channel separation (less crosstalk). While the noise measured 14 dB worse compared to the gainless version it’s still slightly better than the Mini3 (both are fairly quiet subjectively):

Measurement Cmoy 13 dB Gain Cmoy 0 dB Gain AMB Mini3
Frequency Response +/- 0.1 dB Excellent +/- 0.1 dB Excellent +/- 0.1 dB Excellent
THD 1 Khz 150 Ohms 0.002% Excellent 0.001% Excellent 0.002% Excellent
THD 1 Khz 15 Ohms 0.030% Good 0.003% Excellent 0.017% Good
THD 20 hz 15 Ohms 0.028% Good 0.005% Excellent 0.01% Very Good
THD 20 Khz 15 Ohms 0.040% Very Good 0.02% Excellent 0.45% Poor
IMD CCIF 0.010% Good 0.003% Excellent 0.043% Fair
IMD SMPTE 0.012% Good 0.003% Excellent 0.009% Very Good
Noise (ref 400 mV) -89 dB Very Good -96 dB Excellent -94 dB Excellent
Max Output 15 Ohms 100 mW Excellent 67 mW Very Good 104 mW Excellent
Max Output 150 Ohms 180 mW Excellent 180 mW Excellent 38 mW Fair
Output Impedance 0.67 Ohms Very Good 0.1 Ohms Excellent 0.9 Ohms Very Good
Crosstalk 15 Ohms 65 dB Very Good 68 dB Very Good 40 dB Poor
Channel Balance Error 1.1 dB Good 1.1 dB Good 1.14 dB Good
DC Offset 21 mV Good 4.5 mV Excellent 4.1 mV Excellent


  • $39 for a complete portable amp that can drive high impedance headphones is a bargain
  • Respectable distortion performance into 50+ ohms
  • Real ground is free from virtual ground/rail splitter problems
  • DC blocking capacitor on input helps protect headphones
  • Simple design is easy to modify (no surface mount components)


  • Headphones could be damaged if one battery becomes disconnected
  • Headphones could be damaged if one battery dies before the other
  • Lacks AC power option, batteries must be removed for charging
  • Marginal distortion measurements with 16 – 50 ohm loads
  • Limited output current into difficult loads
  • Some channel balance error (cheap pot)
  • DC offset could be marginal with some components
  • No RF protection

BOTTOM LINE: While adding a suitable amount of gain degraded the distortion it’s still respectable—especially considering the price. The distortion is also much less likely to be audible than the Mini3’s virtual ground related distortions. My biggest concern is possible headphone damage if you ignore the warning signs (bad sound) and run mismatched batteries too low (see the original Cmoy article). Or if one battery becomes disconnected (or isn’t connected properly in the first place) the result could be tragic. This design also has some DC offset issues and higher distortion than I would like to see into low impedances. But, overall, this amp is a relative bargain for headphones around 50 ohms and higher. This amp generally matches the bargain FiiO E5 into lower impedance loads but this Cmoy is capable of much higher output voltages for driving higher impedance cans properly. The E5 struggles, for example, with the HD650s but this Cmoy handles them easily.


THE ORIGINAL PROBLEM: eBay-4556-Cmoy-Schematic-one-channel[1]The original Cmoy had a 470 ohm feedback resistor (R1) with 100K to ground on the negative op amp input (R2). This resulted in very nearly 100% feedback and essentially unity gain.The original design is shown in the schematic  to the right.

MODIFICATION: Resistor R1 was increased from 470 ohms to 1.3K to avoid excessive loading and R2 was reduced from 100K to 360 ohms to provide the proper gain. The result can be calculated as follows:

Gain = 1+ (1300/360) = 4.6X = 13.2 dB

This means a typical 500 mV RMS input would yield a healthy 2.3 V RMS output—plenty to drive even full size Sennheisers like my HD650s.  R2 could be as low as about 200 ohms for even more gain if desired or increased for less gain.

IT’S A FLOOR WAX AND A DESSERT TOPPING: In a classic Cmoy design, such as this one, there’s just one op amp to do all the work. In the gainless Cmoy it only had to serve as a unity gain buffer and drive the headphones. With gain, however, it has to serve as the gain stage and also as the output stage. That’s less than ideal as most of the distortion is generated in the output stage but now there’s substantially less feedback to correct that distortion. It’s sort of like a front wheel drive car where the same set of tires have to provide the steering and put all the power to the pavement. It’s a compromise which is why the highest performance cars are rear wheel, or all wheel, drive. And there are more problems with having one op amp do everything. Such as DC offset.

DC OFFSET: Op amps, especially ones with bipolar inputs like the 4556, often have a significant amount of input bias current. In this design, the bias current flows through R3. The datasheet lists the current as 50 nA typical and 500 nA worst case. So V=I*R gives 50nA*100K Ohms = 5 mV. With a gain of 1 that yields 5 mV DC offset on the output. I measured about 4 mV. With a gain of 4.6, however, 5 mV becomes 23 mV and I measured about 21 mV. But the worst case number of 50 mV becomes 230 mV or nearly 0.25 volts. 23 mV is not much of a problem but 230 mV is too high. This is another downside to having a single op amp do everything. Less gain clearly helps here so, especially in a design like this, you only want as much gain as you really need.

BIAS CURRENT TRADEOFFS: You can lower the value of R3 to reduce the offset voltage. But that compromises the –3 dB low frequency roll off caused by C5 and R3 and, if you lower it too much, will hurt the bass response and create more phase shift. R3 could be lowered to 22K and the –3 dB point would still be plenty low with a 2.2 uF capacitor. That would improve the DC offset by a factor of 5 and is worth doing. So it would be around 2 mv – 5 mV typical which is fine but worst case could be as high as 50 mV which is marginal.  Trying to improve it further, however, involves more compromises. If you swap the 4556 for an op amp with less bias current you will have a hard time finding one with similar levels of output current (the main strength of the 4556). Even the expensive AD8397 used in the Mini3 has similar bias current specs to the 4556. So you would give up output current for lower DC offset—probably not a good trade off. You could make C5 bigger, and then R3 can be even lower, but a much bigger film cap is expensive and won’t fit in the tiny Cmoy enclosure so you would have to use an electrolytic which have more distortion. It also lowers the input impedance and changes how the volume control works. So 22K, or possibly 10K, is probably about as low as you want to go with R3.

NO GAIN VS GAIN: The graphs below tell most of the story with respect to what adding gain does to the distortion performance.  The short version is, into low impedance loads, there’s about 10 times more distortion. But the distortion was so low to begin with, this isn’t as bad as it sounds. The details can be found below.

THD+N vs OUTPUT 15 OHMS: The bad news is there’s about 10 times more distortion into 15 ohms at all power levels. The good news is it’s still fairly modest below about 0.03% and remains fairly flat until clipping versus the steeply rising distortion of the Mini3 into the same load. Shown below is the original amp without gain in red and the modified amp with 13 dB gain in blue. The slightly higher clipping voltage is due to the batteries being more fully charged not the gain change. It hit 100 mW at 1% THD:

eBay 4556 Cmoy 1 Khz THD N vs Output 15 Ohms 13 dB Gain (blue) 0 dB Gain (red)

THD+N vs OUTPUT 33 OHMS: Here’s the difference at 33 ohms. At typical listening levels for 33 ohm headphones (i.e. < 1 V RMS) the distortion is still below 0.01% (i.e. the distortion products are all below –80 dB). This also outperforms the Mini3 (again, the higher clipping voltage is due to fully charged batteries):

eBay 4556 Cmoy 1 Khz THD N vs Output 33 Ohms 13 dB Gain (blue) 0 dB Gain (yellow)

THD+N vs OUTPUT 150 OHMS: This is as above but into 150 ohms. This time blue is the original and yellow shows the result with 13 dB of gain. Even at 2 volts RMS the distortion is still very low at about 0.004% but it is higher across the board as expected:

eBay 4556 Cmoy 1 Khz THD N vs Output 150 Ohms 13 dB Gain (yellow) 0 dB Gain (blue)

THD vs FREQUENCY 15 OHMS: This compares the the gain version (blue) to the no gain version (yellow) into 15 ohms from 20hz to 20 Khz. Again, the distortion is about 5 – 10 times higher but still impressively consistent across the band and still relatively modest (the droop above 10 Khz is due to the analyzer bandwidth used):

eBay 4556 Cmoy THD N vs Frequency 15 Ohms 13 dB Gain (blue) 0 dB Gain (yellow)

CCIF IMD 15 OHMS: The CCIF IMD shows higher sidebands around the high frequency signals up to almost –70 dB. This is still decent performance, and still better than the Mini3 in some ways, but notably worse than the no gain version:

eBay 4556 Gain Cmoy CCIF IMD 15 Ohms Ref 400 mV

SMPTE IMD 15 OHMS: This is also higher, as expected, but still a decent result considering the 15 ohm load. The relatively benign 2nd harmonic of the 60 hz tone (which is not considered IMD) is over –80 dB but everything else is comfortably under –80 dB:

eBay 4556 Gain Cmoy SMPTE IMD 15 Ohms Ref 400 mV

GAIN & SPECTRUM: Here’s the gain and spectrum at 1 volt out into 100K at 1 Khz:

eBay 4556 Cmoy 13.3 dB Gain 216 mV in 1 V out 100K (ref 1V RMS)

NOISE: As expected, the noise performance is much more modest but still very respectable. The 60hz hum component is likely due to loop area on the PCB layout and is limiting the unweighted measurement. This hiss is mostly below –130 dB making it subjectively very quiet:

eBay 4556 Cmoy 13 dB Gain 1 Khz - 80 dBr Noise (ref 400 mV)

OUTPUT IMPEDANCE: 400 mV no load (100K) at 1 Khz dropped to 383 mV which gives an output impedance of 0.67 ohms. With no gain, it measured about 0.1 ohms. The higher impedance is due to less feedback and the relatively high inherent (open loop) output impedance of the 4556 op amp. Anything under 2 ohms is fine so this still isn’t a problem.

CHANNEL SEPARATION (CROSSTALK): The crosstalk was a few dB worse but was still excellent at around 80 dB into 150 ohms and better than 60 dB into 15 ohms. The dominant factor, by far, is the output jack and ground traces on the PCB. So changing the gain didn’t make much difference.

CHANNEL BALANCE: This was unchanged as it’s related to relative errors in the volume pot and is independent of gain.

PHASE: The phase response was unchanged and still excellent.

SQUARE WAVE RESPONSE: The square wave performance was still excellent and the slew rate remained faster than 3 V/uS.

TECH SECTION SUMMARY: While the distortion and noise numbers have gone from being fairly amazing to merely decent this is still a respectable performance for $39. But there are some concerns. My particular 4556 has about 20 mV of DC offset in this configuration which is fairly high but still acceptable. But the worst case values on the datasheet could yield much worse. So I would suggest measuring the offset of any similar 4556 Cmoy. If need be, R3 can be lowered to 22K or even 10K as explained under DC Offset at the start of the Tech Section. It’s also worth noting not everyone needs 13 dB (4.5X) of gain. If you can live with say 2X (6 dB) gain with your source and headphones, you would get significantly better performance (it would roughly split the difference between the no gain version and the 13 dB gain measurements). At higher than 13 dB gain the distortion would start to be more of a concern with this single stage design. As explained earlier, you can only expect so much of something that’s a floor wax and a desert topping.


  1. Now if only more of the people selling cmoys on ebay spec'd their stuff like that.

    Most I've seen just brag about using expensive parts or compete on price with few details.

  2. I agree Maverick. Based on some of what I've seen, I doubt many eBay headphone amp/DAC sellers even have the equipment to make proper measurements. Their products often have flaws that just don't make any sense. Like headphone amps with no voltage gain, grounding completely wrong on PCB layouts, DAC chips implemented incorrectly, etc. But it just takes one positive subjective review on say Head-Fi and people pile onto the broken bandwagon and buy stuff that really has no right even being sold.

    I suspect some sellers are blissfully unaware their designs perform horribly while others probably had higher hopes for their products but were forced to dump them on eBay once the serious flaws were discovered. It's one thing for someone to throw together their own DIY project on the kitchen table unaware it performs nothing like it should. But selling such products to the gullible audiophile community is just wrong. Sadly, because of the magic powers of subjective bias, many get away with it.

  3. One more week you delivered a really nice review. It would be cool if you could pass through your lab an iBasso T3 ;)

  4. Thanks Gonzalo. I'm not sure about testing the 3 channel T3 as it likely suffers some of the same problems as the AMB Mini3. See my 3 Channel Article for more. But that said, it does seem to have some useful advantages over the Mini3 including a LiPoly battery, selectable gain, USB charging, and it costs about 50% less.

  5. the best blog for audiophile!!!!!
    review for FIIO E11 PLEEEEEASEEEEEE!!!

  6. Ups, did not know anything about the 3ch architecture.Going by the numbers the iBasso T3 seems to measure quite well but obviously i'm referring to a few RMAA's. Subjectively i'd say it's competent and clean for multidriver iems.

  7. Thanks for the follow up. Its too bad you cant buy this exact amp on ebay any more as it would be a great cheap way to drive high z phones. I would easily take one over a mini3.

    As an engineer I agree with your 3 channel design view. I can see it where you have no other choice but to claim it's superior to a conventional ground is pure audiophile BS. The 3 ch fanbois should get their heads out of the sand and realize it's, at best, a serious compromise.

    But I can sympathize. A buddy does cryogenic freezing of his opamps claiming way better sound. I just smile and nod like when a 5 year old explains he just got back from a trip to outer space.

  8. As just a fan of the most accurate sound i can get into my multidriver UM3x iem i'm kind of a fan of the iBasso T3 as a way of getting a ruler flat response into the transducer without degrading other aspects like THD or crosstalk. What i did not know till this day was that flaws of the 3ch concept may render not so good crosstalk figures.

    Here you can check an RMAA on an iPod Nano with a T3 hooked.


    And without the T3D:


    Is it possible to draw any conclusion from here ?

    Subjectively i do appreciate audibly more treble detail with the T3 and a pinch more bass when hooking it to my iPod Classic.

  9. To Gonzalo, I have my doubts about RMAA's crosstalk measurements in general. But, if the tests you link were really loaded with the UE TripleFi headphones, the iBasso does at least seem to have a very low output impedance (which is good). According to those tests, the Nano 6G does roll off at high frequencies in a potentially audible way due to its higher output impedance.

    The crosstalk is not, in practice, the greatest issue with the AMB Mini3. It's the added distortion and the nature of the distortion. The shared ground channel is not perfect. So each audio channel creates distortion components on the output of the ground channel. And those are shared between the channels. With stereo music, you get distortion artifacts in the right channel related to the music in the left channel and visa versa. This inter-channel distortion is not something that's normally measured (including by RMAA) because it essentially doesn't exist with a conventional ground. But it's likely a more audible form of distortion than the usual single channel THD.

    To D Wilson, thanks for your comments. I've heard the cryogenic hype but it's been more for passive components like cables, speaker crossovers, etc. I've never heard of freezing op amps for better sound.

    To Anon, yes I may review a FiiO E11 one of these days. It's apparently a rather similar design to the Mini3 so it would be interesting to see how FiiO's engineers stack up against a non-engineer DIYer like Ti Kan at AMB given more or less the same ingredients--sort of a battle of the Chefs.

  10. "given more or less the same ingredients--sort of a battle of the Chefs"

    So who's "Iron Chef: Silicon" and does that count as an oxymoron?

  11. NwAvGuy, what is your opinion on neco soundlab v2, similar to cmoy, but power supply and the channels audio separate..

    thanks, you're the best

  12. The Neco Soundlab V2 is a single battery design and uses the TLE2426 rail splitter which is only rated for 20 mA of current. That's only 10 mA per channel and LOTS of headphones need much more than 10 mA even some common ones like the Denon AH-D2000s. So for anything but very efficient high impedance ( > 250 ohms) headphones, the Neco Soundlab is a poor choice.

    The TLE2426 will run out of current and the amplifier will distort badly with many headphones. There are many similarly flawed Cmoy-like single battery designs and it amazes me they sometimes get such good subjective reviews. For more details about rail splitters, and single battery designs, see: 3 Channel Amplifiers

  13. ok, discard the Neco SoundLab, but i have seen very good reviews...

    which is the best design for headphone amplifier
    DIY? Cmoy with channel separate?


  14. I will be reporting more on DIY designs soon. Having separate channels is questionable as most of these designs are limited in other ways (like the rail splitter on the Nico). In a properly designed amp separate channels can have some measurable advantages. But eBay amps are rarely properly designed.

    A lot of the headphone amps and DACs being sold on eBay are all about buzzwords, gimmicks, marketing hype, myths, using popular audiophile op amps, tubes, fancy looking cases and PC boards, etc. But they're rarely properly tested.

    You end up with only very biased subjective reviews on places like Head-Fi where lots of people think spending $200 on cables is a good idea. They're nearly always wrong about the cables and they're also often wrong about a lot of the eBay products. See Subjective vs Objective.

    In my experience, the cheaper eBay designs nearly always have serious flaws (like the Cmoy with no gain, high distortion, oscillation, etc.). And I suspect a lot of the people making them don't even have the right equipment to properly measure their designs. So without buying 20 different amps to hopefully find a few good ones, it's risky to buy on eBay as returns are usually not practical.

  15. Just was told the RA1 cmoy "clones" are 8-12ohms output impedance. Surprisingly high. Definitely not a true RA1 clone if it's got a split rail and isn't near zero ohm. Too bad, as this one you found on ebay seems very rare. It's probably closer to the Grado than the ones that say RA1 on them on ebay.

  16. FWIW, a few years ago I bought an RA1 clone on ebay. I sold it on head-fi after 2 weeks.
    From a purely subjective stand point: it did mate well with my Grado SR-80, didn't move the Senn HD580 properly. While the sound was not bad at all, I felt my portable pimeta was better.
    I am not sure how close that clone was to the actual RA1 circuit though (didn't occur to me to check). That is to say, the single opamp design has a lot going for... though the scenarios with one battery failing and consequent damage is something that I never even considered - in fact, I think most folks do not expect an amp to fry a headphone, no matter how shady was the place from which it was purchased.

    Question: if one battery on the O2 gets disconnected or dies before the other one, any bad side effects?

  17. I've seen several RA1 clones that use a different op amp, and/or use a single battery with a rail splitter. Neither can be expected to work very well as the 4556 stands alone among all dual DIP8 op amps for driving low impedance loads. Any op amp "upgrades" are really "downgrades".

    The power management circuit in the O2 immediately shuts the amp down with little drama if either battery becomes disconnected or weak. That's a first in any portable headphone amp I'm aware of. It also helps the rechargeable batteries last much longer by helping prevent cell reversal. See: Circuit Description

  18. Hi, It's Daniel again. Got another question for you. I got a JDS Labs Cmoy PCB Laying around, would it be worthwhile to complete it and mate it with my AKG701's til I get my hands on a ODAC-combo or something similiar?

    Any ideas on what OPamps to use for it? 4556?

  19. @Daniel, the K701 needs 3.2 Vrms that's 4.5 volts peak. At 62 ohms, that's 72 mA of peak current. The JDS designed amp can only manage about 20 mA of peak current because of the ground splitter used for the virtual ground.

    So, in summary, the JDS amp is a poor match for the K701s. Putting in a high current op amp like the 4556 won't help because the current output is severely limited by the virtual ground design. See my More Power article for how to figure out what headphones will work well with only 20 mA of peak current.

  20. You mention that with your eBay cmoy, "There’s a “pop” when you first turn it on that’s not that bad—especially with more typical headphones. There’s a softer click when you turn it off." After reading the back story to the Schiit Asgard being flagged as potentially dangerous to headphones during power-off, I've become someone concerned if other amps without a protection relay system can be detrimental to headphones after many cycles. Specifically, I have an older CMOY (CHA-47 variant) that gives an audible 'click' when powered on and a low-frequency "thump" when powering off. Is there reason for concern that I could be doing my headphones a disservice by the on/off cycle of my amp? And to mitigate this is it an acceptable solution to 1st power on with no load on the headphone output, then plug in my headphones (and vice versa before powering off) -- or is it bad for the amp to power on/off with no load on the output?

    1. It's not a black and white thing in most cases, although the Asgard was way over the top and obviously a hazard (as confirmed by AKG and video of the driver being severely deformed). Towards the end of my O2 Details article I document the turn and turn off transients of the O2. In both cases they're no greater than what an iPod routinely puts out playing music. The turn on click is very brief so it contains little energy. The turn off "thump" approximates a half cycle of a typical bass note in music. In both cases the peak level is below what even an iPod can manage into the same load while playing typical music.

      Without using digital scope capable of capturing the transients into your headphone load, it's hard to know what's "too great". And some headphones are certainly more fragile than others. If I had a pair of $1000 Westone IEMs, for example, I would probably pay extra attention to transients. Comparatively, my HD650s are probably fairly immune to any reasonable transient.

      It's well down the list of future article topics, but I think it would be interesting to take a couple of modestly priced headphones (something like the Sennheiser CX300 and HD201) and first measure them using the dScope for frequency response, resonance/impedance, distortion, etc. Then "abuse" them in various ways including progressively greater DC offsets, larger transients, etc. and see if the performance changes.

      I have several arbitrary waveform generators that are capable of "cloning" the Asgard's transient, or any other transient, and then repeating it as often as desired for as long as desired to simulate extended use. At various points along the way the headphones could be re-tested to see if their performance had changed.

      Such testing is relatively basic but I'm not aware of anyone who's done it in a controlled way and published credible results. The downside is what the CX300 and HD201 survive may still harm those $1000 IEMs. But it would at least help establish some thresholds.

      On a more general level, if you're using expensive headphones with a typical Cmoy you may be running several risks beyond just the transients--namely serious DC at the output if anything goes wrong with the power supply, etc.

  21. I have a couple of questions regarding the DC offset issue.

    I tried to measure the DC offset of one of my sources. What I did was to hook up a 3.5mm interconnect to the headphone jack. Then I took a multimeter, set it to DC 200mV and started playing music at the volume level which I listen to.

    I connected the black lead to the Ground and the red lead to the Left channel. Initially I got a reading of 91mV. But the longer I held the leads in place, the more this value decreased. Within twenty seconds the value got down to 89mV.

    So here are my questions.
    Is whatever I'm measuring really the DC offset of the source?
    If so, is 91mV into headphones a bit too much?
    Also, why does the value decrease (rather quickly at that) over time?

    I have to say that I didn't hook up any loads in serial and I don't know the input resistance of the meter.

    Thank you very much!

  22. Anything over 20 mV may be cause for concern depending on the headphones. But you have two issues that might be effecting your reading. First, yes the output should be loaded--you can use a "Y" splitter cable and plug headphones into the other jack or you can use a resistor in the 32 - 600 ohm range in parallel with the DMM. Second, no music should be playing. The DMM is likely to get confused by the AC signal (especially deep bass notes). Because some devices, including iPods, shut off their output amps when nothing is playing it's best to play music at the lowest possible volume setting where you can still hear the music in the headphones.

    1. Good news.

      I tested again using this correct method, with a headphone splitter and no music playing.
      I am reading exactly 0.00 mV (always 200 mV scale). I even swapped the two sockets (on the splitter) to make sure things are working right.
      I do indeed hear a faint hiss in my IEMs when plugged in so the output circuit is on. I have encountered devices that shut down their output circuits when nothing is playing. Those devices emit zero hiss.

      So any ideas on why I previously read those millivolts, testing unloaded?

      Thank you very much!

    2. It sounds like your device may have capacitor coupled outputs. With only the load of the DMM (likely 1M--1 million ohms) there will be some DC "leakage" of the caps causing somewhat random readings. That's normal. The charge current of the caps also has an exponential asymptotic decay which means a high impedance load equates to a long time before it gets so close to zero your DMM will no longer measure it.

    3. I see. I sorta anticipated this capacitor-coupled output thing.
      I should go measure my MAX9722 (direct coupled, no caps) and see what comes out of it.

    4. Would you know the reason behind the fact that few portable players are direct-coupled? Is it due to the high degree of integration (everything audio in a single SigmaTel)? Is capacitor coupling any cheaper? That wouldn't make any sense since even the Clip+ is direct-coupled.

    5. I believe the Clip+ uses, essentially, a virtual ground to avoid needing output capacitors (see my Virtual Ground article from last May). In something as small as the Clip+ space for the capacitors could be a serious concern. To get reasonably decent low frequency performance into 16 ohm headphones you need at least 470 uF caps which are physically fairly large.

      But, in general, using electrolytic coupling caps is the cheapest solution using off the shelf parts. But when you're Apple, Creative, or Sandisk, you have more options available than the usual off-the-shelf components--such as highly integrated "SOCs" (System On a Chip) as used in the Clip+.

  23. So, I got one of these to play with and used your info to experiment a bit. I know there's debate on what can be heard but I used my ear and bits on hand to see what I could come up with. Damping factor and things like TIM can be audible even if not measured. For those that want to do the same please not that resistor positions were marked differently on mine, a fanmusic unit that was otherwise identical and actually of surprising quality.

    The 22k was a good call to lower offset. 10k wasn't as solid or fast sounding. Used a 1k resistor as it was here instead if the 1.3K and a 560 in the last spot for about 2x gain. I prefer this to the very slightly dead sounding unity gain and I don't need more so offset is nicely controlled. Tried some bypass caps on the electrolytic. Tants in this case and the smaller values weren't overall favorable though they did seem to clean it up very slightly. Tried some 100mf 10V as bypass and these worked well for some reason. Subtle and all including using none were good. Input caps are Tants with + at the input side. Theoretically not ideal but I had a few different 10mf types around and one set is actually very good there. I've had good results with chosen polar tants in the past but it's a bit of hit and miss outside of a feedback loop. End result is that this thing is clearly better than an E11 for instance and with less than 3mv of DC. Neutral, dynamic and actually more finesse than I thought a 4556 could have. Testing was in series with a low impedance amp as source and then just some more listening.

    I suspect it's all about balancing parameters and using devices withing their design. I haven't measured a thing besides offset but your guidelines put me in good stead. I tried and quickly sold a fiio E11. It wasn't bad but not special either. I have never liked the idea of 3 channel amps but it appeared the way to go until now. I also considered building one of those 5532 inverting amps with a single rail but I also love that I don't need output coupling caps without added complication. I have to thank you for this great read.

  24. To add, I ended up with 47K at R1 and 22k at r2 and r3 for about 4db gain and the offset averages 1mv per channel.


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